Have CSSAnimationData::name() / initialAnimationName() return an AtomicString

Have CSSAnimationData::name() / initialAnimationName() return an AtomicString
instead of a String. Most callers require an AtomicString and needed to
convert the returned String to an AtomicString. Also note that m_name is
stored as an AtomicString anyway and setName() already takes an AtomicString
in argument.

R=abarth
BUG=323739

Review URL: https://codereview.chromium.org/90033004

git-svn-id: svn://svn.chromium.org/blink/trunk@162768 bbb929c8-8fbe-4397-9dbb-9b2b20218538